The present invention relates generally to an in-line drying and extrusion system and, more particularly, to an in-line drying and processing system that produces net shapes from cellulosic compounds. The present invention is useful for many different formulations and material composites which will become apparent to one of ordinary skill in the art upon reading this disclosure. For example, the present invention may be used in conjunction with cellulosic/polymer composites that include polyvinyl chloride (PVC) or high density polyethylene (HDPE).
Cellulosic compounds, e.g., synthetic wood compositions, have been advanced as a substitute for natural wood. Synthetic wood compositions are typically made by combining cellulosic materials and thermoplastic materials. Many prior attempts to manufacture synthetic wood components have failed due to the excessive moisture content of the cellulosic material. For instance, water from the cellulosic material can flash from the surface of an extruded component as a result of rapid devolitalization. The water can also cause steam bubbles to pass from the interior to the exterior of an extruded component thereby leaving a substantial flaw. In addition, surface water can cause cracks, bubbles, and other surface flaws. As a result, cellulosic material having excessive moisture content can compromise the aesthetic and structural quality of an extruded synthetic wood component.
Cellulosic materials have a natural moisture content. For example, some freshly cut trees may have a water content of 30% to 300% by weight based on fiber content. The moisture content of the wood typically decreases as a result of processing, e.g., the making of sawdust. However, the moisture content will still be excessive for many known manufacturing processes if additional steps are not taken to further dry the cellulosic material prior to mixing it with the other compound materials. In addition, these known manufacturing processes typically require additional steps to maintain the low moisture content of the cellulosic material prior to mixing it with the other compound materials. For instance, these known manufacturing processes may require the cellulosic material to be stored in a humidity-controlled environment after it has been dried to a desired level. Otherwise, the cellulosic material will collect additional moisture from the environment.
Known manufacturing processes may also possess other shortcomings. Some known manufacturing processes attempt to use a single heated screw extruder to simultaneously mix and dry the compound materials. Such processes typically require the measured addition of the compound materials to the extruder via heated feeders. Thereafter, the extruder may use high heat to blend the compound materials into a composite melt. However, the cellulosic materials may still not be uniformly dried in the extruder, and the high heat may cause the thermal degradation of the cellulosic materials. In addition, the composite melt may have to be subjected to a vacuum as it passes through the extruder in order to improve the drying of the cellulosic materials.
In light of the shortcomings of known manufacturing processes and systems for cellulosic compounds, a need exists for an improved process and system wherein the compound materials may be stored in a humid environment prior to being introduced the system. Another need exists for an improved process and system wherein the compound materials may be pre-mixed and then stored in a humid environment prior to being introduced to the system. A need also exists for an improved process and system that uses pre-mixed compound materials and does not require the measured addition of the compound materials at any point during the processing. There is also a need for an improved system that utilizes a drying means that operates at cooler temperatures than the drying means of known systems. In addition, a need exists for an improved system that does not need heated feeders to transfer the compound materials to the extruder. Finally, a need exists for an improved system in which the compound materials do not have to be subjected to a vacuum while being processed through the extruder.
The present invention satisfies some or all of these needs. A preferred embodiment of the present invention is a process for drying and extruding a cellulosic compound that is comprised of predetermined materials. The process begins by providing the predetermined materials in a desired ratio. The predetermined materials are then mixed at a first location. Thereafter, the predetermined materials can be stored until needed. When it is time to extrude the predetermined materials, the predetermined materials are transferred to a dryer at a second location remote from the first location. The dryer has a surface area enabling the predetermined materials to be sufficiently spread out for drying. The predetermined materials are substantially uniformly dried to a desired level in the dryer. The predetermined materials are then extruded to obtain a final net shape.
The predetermined materials may include at least one thermoplastic material. For example, the thermoplastic material may be polyvinyl chloride, high density polyethylene, polypropylene, other suitable thermoplastics, or combinations thereof. The predetermined materials may also include wood flour, wood fiber, or other suitable cellulosic materials.
The predetermined materials are preferably mixed using a ribbon blender. In addition, the dryer is preferably a vertical rotary tray dryer. The dryer preferably maintains the drying temperature between about 250 and about 350 degrees Fahrenheit.
The present invention also includes a system for drying and extruding a cellulosic compound comprised of predetermined materials, wherein the predetermined materials have already been mixed together in a desired ratio. The system comprises a dryer and an extruder. The dryer has a surface area which allows the predetermined materials to be sufficiently spread out for drying. Consequently, the dryer is adapted to substantially uniformly dry the predetermined materials to a desired level. The extruder is connected to the dryer. The extruder is adapted to receive the predetermined materials from the dryer. The extruder is then adapted to force the predetermined materials through at least one shaping device to obtain a final net shape.
The dryer may be a vertical rotary tray dryer or any other suitable type of dryer. The dryer is preferably comprised of at least one rotating drying tray. It is preferred that the dryer is adapted to substantially continuously receive the predetermined materials. It is further preferred that the predetermined materials are substantially continuously transferred from the dryer to the extruder.
The present invention includes another embodiment of a system for drying and extruding a cellulosic compound comprised of predetermined materials, wherein the predetermined materials are provided in a desired ratio. The system comprises a mixer, a dryer, and an extruder. The mixer is at a first location, and it is adapted to mix the predetermined materials together. The dryer is at a second location remote from the first location. The dryer is adapted to receive the predetermined materials that have been mixed together. The dry has a surface area enabling the predetermined materials to be sufficiently spread out for drying. As a result, the dryer is adapted to substantially uniformly dry the predetermined materials to a desired level. The extruder is connected to the dryer. The extruder is adapted to receive the predetermined materials from the dryer. After receiving the predetermined materials, the extruder is adapted to force the predetermined materials through at least one shaping device to obtain a final net shape.
This system may include any of the optional or preferred features of the above-described system. In addition, the mixer may be a ribbon blender or another suitable type of mixer. It should be recognized that the mixer may be heated.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments.